Interpretive Summary: Carbohydrate movement and utilization in plants is key to plant survival, growth and development. The primary form of carbohydrate which is transported within a plant is sucrose. Sucrose is principally produced in the leaves as a product of photosynthesis and transported to other parts of the plants such as the stems, roots, flowers, etc., where it is broken down into glucose and fructose for use as an energy source. Therefore, the process by which sucrose is broken down is instrumental to supplying energy to non-photosynthetic cells. This paper looks at the effect of the over-expression of two of the major enzymes involved in sucrose and carbohydrate metabolism, UDP pyrophosphorylase (UGPase) and Sucrose Synthase (SuSy), on plant growth and cell wall formation in the stem. The hypothesis underlying the research was that the stem is a storage organ, similar to a potato tuber. It has been shown in potatoes that the manipulation of carbohydrate pools within the tubers can affect starch content. Since both starch and cellulose, the primary component in plant cell walls, are in essence long chains of glucose residues it was hypothesized that manipulation of cellulose content in cell walls could also be done by the manipulation of carbohydrate pools in stems. In this study, the genes encoding these two enzymes were over-expressed singly or together in transgenic tobacco. The over-expression of these genes were controlled in two ways, in one set of transformants, the genes were over-expressed in all plant parts, while in another set of transformants the genes were over-expressed only in the stems. Interestingly, the over-expression of either or both enzymes did not have much effect on cellulose content or fibre quality in the stem. As well, there was not much change in plant growth with only one enzyme expressed regardless of the location of expression. However, when genes encoding both enzymes are over-expressed, significant differences in plant height were observed. These results confirm the strong role of sucrose metabolism on plant growth and suggest that alteration of these two enzymes could be used to increase plant growth.

Technical Abstract:
The effects of over expressing cotton (Gossypium hyrisutum) sucrose synthase (SuSy) and Acetobacter xylinum UDP-glucose pyrophosphorylase (UGPase) genes on plant growth and metabolism were evaluated in tobacco (Nicotiana tabacum cv. Xanthi). T1 transgenic tobacco plants expressing either gene under the control of a tandem repeat of the Cauliflower Mosaic Virus 35S promoter (2X35S) or a xylem-localized 4CL promoter (Petroselinum crispum 4-coumarate:CoA ligase promoter; 4CL) were generated. Plant carrying single transgenes were further reciprocally crossed to generate T2 double transgene plants, expressing both the SuSy and UGPase genes. Transcript levels, enzyme activity, growth parameters, fibre properties and both structural and soluble carbohydrates of stem tissue of all transformed lines were quantified. Expression profiles of both genes confirmed the expression pattern of the promoters: 2X35S being expressed more strongly in leaves, while 4CL expression was highest in stem tissue. In depth plant characterization revealed that the single transgene lines had significant increases in height growth compared to the corresponding control lines. The double transgene plants demonstrated an additive effect where these plants proved to be even taller than the single transgene parents. Several of these lines had associated increases in soluble sugar content. Fibre characteristics were also consistent with enhanced growth rates, as the plant cells were slightly shorter and had thicker cell walls, representative of reaction wood formation. While no partitioning of storage of carbohydrates into starch or cellulose was observed, the increased growth rate, coupled with the increases in hexose sugar concentrations, suggest a role for SuSy as a marker in sink strength and further lends credit to the function of UGPase in a similar role. The up-regulation of these two genes, while not increasing percent cellulose content, was effective in increasing total plant biomass, and thus the overall yield of cellulose attainable from a given plant.